One of the most sensitive and accurate versions of the vortex flowmeter can be constructed by using a highly sensitive oscillatory pressure sensor, that detects the oscillating differential pressure across a plane of symmetry of a vortex generating bluff body. When the fluid velocity becomes very low, the amplitude of the alternating differential pressure oscillation associated with the vortex shedding from the vortex generating bluff body becomes extremely low and, consequently, the measurement of the fluid velocity as a function of the vortex shedding frequency encounters a serious obstacle arising from the difficulty in counting the vortex shedding frequency accurately. In theory, the amplitude of an alternating electrical signal generated by an oscillatory pressure sensor or transducer can be brought up to any desirable level by supplying the two fluid pressures respectively existing on the two opposite cylindrical sides of the vortex generating bluff body into two pressure compartments separated from one another by a thin deflective partitioning wall of a large surface area, and by detecting the force or deflection experienced by the thin deflective partitioning wall. The parent patent and patent applications have disclosed a number of pioneering and highly successful versions of the vortex flowmeter and the oscillatory pressure transducers operating on the above-described theory.